Normally alternating current (AC) track circuits using vane type relays as track relays are energized by steady energy, both the track and local windings. In some installations, such track circuits also provide coded AC energy in the rails when a train occupies the section to control cab signal and/or other train carried apparatus. Under certain operating situations, e.g., when a train backs out of the section or a temporary shunt is removed, the track circuit must reset to its normal at-rest, steady energy condition from the coded track energy condition. This requires the vane relay to receive sufficient energy to at least open its back contacts, to release the cab signal energy control relay. One way to assure this level of energization is to increase the local and/or track supply voltages when a train occupies the section. This is acceptable where a standard track shunting sensitivity is used, i.e., track circuit detects a minimum 0.06 ohm train shunt. However, some systems use higher shunt levels, e.g., 0.5 ohm shunting sensitivity. Under such higher limits, increasing the local or track voltage levels for reset is not possible, since this reduces shunting sensitivity to the point that the track relay may not detect the train shunt if it decreases to near the selected minimum level. System safety then suffers. It has been found that a conventional vane type track relay will respond to lower track code rates, e.g., 50 or 75 pulses per minute, but will not receive sufficient energy to open back contacts at higher rates, e.g., 180, 270, 420 pulses per minute, i.e., 3, 4.5, and 7 Hz. Some additional arrangement must therefore be used to assure reset of the track circuit to steady energy.
Accordingly, an object of my invention is a circuit arrangement which assures the reset of an AC track circuit to a steady energy condition following a period of coded rail energy.
Another object of the invention is apparatus for an AC track circuit using a vane type track relay to assure reset of the track circuit and relay from a coded track energy condition to a steady energy condition.
It is also an object of the invention to provide, in a normally steady energy AC track circuit, a pulse filler network at the transmitter location which periodically fills in, with transmitted energy, an off-period of the coded energy transmitted through the rails when a train occupies the section, to sufficiently energize a vane type track relay, during the first such extended energy cycle after a train clears the section, to reset the track circuit to its steady energy state.
A further object of the invention is a code transmitter arrangement for an AC track circuit, which is actuated when a train occupies the section, and includes a logic network to periodically, at a selected time interval, fill in a code off-time period to assure sufficient energization of the track relay after a train clears to reset the track circuit to its normal, at-rest steady energy condition.
Yet another object of my invention is a pulse filler arrangement which is coupled to the code transmitter of an AC track circuit, actuated when a train occupies the section for driving a magnetic stick relay to vitally code the AC energy applied to the rails at a preferred duty cycle to provide cab signal control energy, and which periodically holds that stick relay in position to fill a normal code off-time with transmitted energy to sufficiently energize the track relay to reset the track circuit to its steady energy condition.
A still further object of the invention is an AC track circuit arrangement, normally steadily energized and in which coded track energy is substituted when a train occupies the track section, including a logic network, with a counter, actuated by a code transmitter to drive a magnetic stick relay to code the energy applied to the rails, the logic network responding to a selected count of the code pulses produced by the code transmitter to periodically ridge a code-off period to hold the magnetic stick relay in position to extend the code-on period to supply sufficient energy to pick up a vane type track relay after a train clears the section to reset the track circuit to its steady energy condition.
Other objects, features, and advantages of the invention will become apparent from the following specification and appended claims when taken in connection with the accompanying drawings.